[bluez] Allow telephony feature configuration. Contributes to JB#8808.

Allow user to mark telephony features as disabled by configuration entry in audio.conf E.g. 

    [Telephony]
    Disable=ThreeWayCalling

At the moment this only affects the features AG advertises to headsets, the implementing code is not affected by the configuration. Thus care needs to be taken that the features not advertised to headsets are not used. Modifying the implementation to hedd the configuration would require additional commits.